GJ 1214 b (sometimes called Gliese 1214 b[8]) is an exoplanet that orbits the star GJ 1214. The parent star is 42 light-years from the Sun, in the constellation Ophiuchus. The planet was discovered in December 2009. It is a super-Earth because it is larger than Earth but has a mass and radius significantly less than those of the gas giants in the Solar System. After COROT-7b, it was the second such planet to be known[6] and is the first of a new class of planets with small size and relatively low density.[9] GJ 1214 b is also significant because its parent star is relatively near the Sun and because it transits (crosses in front of) that parent star, which allows the planet's atmosphere to be studied using spectroscopic technologies.[6]

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GJ 1214 b was first detected by the MEarth Project, which searches for the small drops in brightness that can occur when an orbiting planet briefly passes in front of its parent star. In early 2009, the astronomers running the project noticed that the star GJ 1214 appeared to show drops in brightness of that sort. They then observed the star more closely and confirmed that it dimmed by roughly 1.5% every 1.58 days. Follow-up radial-velocity measurements were then made with the HARPS spectrograph on the ESO's 3.6-meter telescope at La Silla, Chile; those measurements succeeded in providing independent evidence for the reality of the planet. A paper was then published in Nature announcing the planet and giving estimates of its mass, radius, and orbital parameters.[6]

GJ 1214 b is about 2.7 times Earth's diameter and is almost seven times as massive.[14]

The radius of GJ 1214 b can be inferred from the amount of dimming seen when the planet crosses in front of its parent star as viewed from Earth. The mass of the planet can be inferred from sensitive observations of the parent star's radial velocity, measured through small shifts in stellar spectral lines due to the Doppler effect.[6] Given the planet's mass and radius, its density can be calculated. Through a comparison with theoretical models, the density in turn provides limited but highly useful information about the composition and structure of the planet.[6]

On that basis,[15] it has been suggested that GJ 1214 b has a relatively thick gaseous envelope.[9] It is possible to propose structures by assuming different compositions, guided by scenarios for the formation and evolution of the planet.[9] GJ 1214 b could be a rocky planet with an outgassed hydrogen-rich atmosphere, a mini-Neptune, or an ocean planet.[9] If it is a water world, it could possibly be thought of as a bigger and hotter version of Jupiter's Galilean moons.[9] If GJ 1214 b is assumed to be an ocean planet,[15] i.e. the interior is assumed to be composed primarily of a water core surrounded by more water, proportions of the total mass consistent with the mass and radius are ~25% rock and ~75% water, covered by a thick envelope of gases such as hydrogen and helium (~0.05%).[6][16] Water planets could result from inward planetary migration and originate as protoplanets that formed from volatile ice-rich material beyond the snow-line but that never attained masses sufficient to accrete large amounts of H/He nebular gas.[9] Because of the varying pressure at depth, models of a water world include "steam, liquid, superfluid, high-pressure ices, and plasma phases" of water.[9] Some of the solid-phase water could be in the form of ice VII.[16]

This artist's impression shows how GJ 1214 b may look as it transits its parent star. It is the second super-Earth for which astronomers have determined the mass and radius, giving vital clues about its structure.

Due to the relatively small size of GJ 1214 b's parent star, it is feasible to perform spectroscopic observations during planetary transits. By comparing the observed spectrum before and during transits, the spectrum of the planetary atmosphere can be inferred. In December 2010, a study was published showing the spectrum to be largely featureless over the wavelength range of 750–1000 nm. Because a thick and cloud-free hydrogen-rich atmosphere would have produced detectable spectral features, such an atmosphere appears to be ruled out. Although no clear signs were observed of water vapor or any other molecule, the authors of the study believe the planet may have an atmosphere composed mainly of water vapor. Another possibility is that there may be a thick layer of high clouds, which absorbs the starlight.[17] Further observations are necessary to determine the composition of its atmosphere.

Because of the estimated old age of the planetary system and the calculated hydrodynamic escape rate of 9×105 kg s−1, scientists conclude that there has been a significant atmospheric loss during the lifetime of the planet and any current atmosphere cannot be primordial.[6]